Transmission control in two-way signaling system



March 17, 1953 Filed Nov. 2, 1950 O..R. GARFIELD TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM 3 Sheets-Sheet 1 7'0 RADIO TRANSMITTER ro 'm0/D RECEIVER NEI' /NVE/vro y 0. l?. GARF/ELD Arrow/Er March I17', 1953 Filed NOV. 2, 1950 O. R. GARFIELD TRANSMISSION CONTROL. IN TWO-WAY SIGNALING SYSTEM 5 sheen-sheet 2 REC.

Kew

ATTORNEY March 17, 1953 o. R. GARFIELD 2,632,052 TRANSMISSION CONTROL IN TWO-WAY SIGNALING sysTEN Filed Nov. 2, 195o 3 sheets-sheet s FIG. 4

SWITCH' BOARD Afro/PNE? Patented Mar. 17, 1953iA UNITED ilivi'iihl OFFICE TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Application November 2, 1950, Serial No. 193,683

4 Claims.

The invention relates to signal wave transmission systems and particularly to arrangements for controlling signal transmission in a two-way telephone system including an intermediate transmission link subject to varying noise or other interference, such as a radio or carrier link.

The invention is specically applicable to, although not limited to, a control terminal for use at each end of a radio telephone system to control speech signal transmission between the radio equipment and an associated two-wire telephone plant. The primary function of such a control terminal is to insure the optimum ratio of useful signal to the noise introduced by the radio link. This purpose is accomplished chieiiy by the use oi speech volume regulation in the two one-way amplifying paths of the terminal respectively outgoing to the radio transmitter and incoming from the radio receiver. Such a terminal may also include a switching device, known as a vodas (Voice-operated device, anti-singing), responsive to the talking currents of the telephone subscribers associated with opposite ends of the system to so control the transmission characteristics of the oppositely-directed amplifying paths of the terminal as to suppress echoes and prevent singing around the four-wire loop formed by these paths, and prevent reradiation of the portion oi the voice signal energy incoming from the radio receiver transmitted into the outgoing amplifying path due to imperfect balance of the hybrid coil arrangement employed in the terminall Proper operation of such a -control terminal also requires the provision of equipment to enable the adjustment of the sensitivities of the transmitting and receiving vodas branches from time to time to take account of varying line and radio noise conditions.

In one type of control terminal heretofore used, the volume regulating and sensitivity controls comprise separate manually operable variable attenuators in the signal transmitting and receiving paths and in the inputs of the two vodas branches, respectively, which are independently adjusted by the technical operator from time to time as service conditions vary. A disadvantage o1" this type of control terminal is the diiculty of determining accurately and quickly the correct adjustment ci the variable attenuator in the signal receiving path, which is a function of the settings of the other three variable attenuators. In another control terminal of the prior art, the required speech signal volume regulation and vodas sensitivities are obtained by an automatic gain adjusting device in the signal transmitting path, separate variable attenuators in the transmitting and receiving vodas branches manually adjusted by the technical operator, and a relatively complex and expensive electricallyoperated control means automatically responsive to these separate adjustments to adjust the loss in the signal receiving path to the proper value which is proportional to the algebraic sum of the effects of the three other controls.

An object oi' the invention is to improve a control terminal of the above-described general type from the standpoint of simpliiication and reduction in the cost of the control equipment.

A related object is to provide with simple and economical equipment, maximum gain in the incoming and outgoing signal transmission paths of a control terminal for a two-way radio or carrier telephone system consistent with the maintenance of anti-singing conditions and discrimination against echoes, line noise and other types of interference such as that arising in a radio path of the system.

Another object is t0 limit the volume level of speech signals received by a telephone subscriber through a control terminal of a two-way radio telephone system to a maximum value which will not be irritating to the ear of that subscriber.

These objects are attained in general by the provision oi suitable mechanical linkages between diierent portions of variable attenuation means in the signal receiving path and each of three separate manual controls for variable attenuators located respectively in the inputs of the signal transmitting path and the vodas transmitting and receiving branches. The three manual controls are separately adjusted by the technical operator to change the loss values of the attenuators directly controlled by them in steps until the transmitted signal volume in the signal transmitting path and. the sensitivities of the two vodas branches are brought to the necessary values for proper operation of the terminal under the existing service condition. This, through the mechanical linkage, will cause the loss values of attenuators in the signal receiving path to be automatically adjusted to make the net loss of that path of the optimum value both to maintain satisfactory echo margin in the terminal and limit the received signal volume at a subscriber station connected to the terminal to the maximum value tolerable to the ear of the subscriber at that station.

In one embodiment of the invention, the shafts of three separate manually rotatable dials operating as the manual controls are respectively mechanically connected to the movable arms of variable resistance attenuators in the signal transmitting path, the input of the vodas transmitting vodas branch and in the signal receiving path in front of the point of connection of the vodas receiving branch to that path in such manner that the technical operator by turning the respective dials in the proper direction may adjust the loss values of the controlled attenuators to bring the transmitted signal volume and the sensitivities of the two vodas branches to the desired values. The shaft of the dial controlling. the variable attenuator in the signal transmitting path and the shaft of the dial controlling the attenuatcr in the vodas transmitting branch are also respectivelycoupled to the movable arms of separate variable attenuators connected in tandem in the signal receiving point beyond the point of connection of the vodas receiving branch to that path, in such manner that the loss values of the latter attenuators are automatically adjusted to produce compensating loss changes in the signal receiving path such as to maintain echo margin for any adjustment of the three dials. A differential gearing arrangement is utilized to effectively interlock the several variable attenuators in the signal receiving path in proper phase relation so that their losses are effectively algebraically added, and other mechanical means including a spring arrangement is utilized to effectively provide slipping between the several interlocked attenuators in the signal receiving path over the high received volume portion of the adjustedrangel so that-the net loss in that path is made such as to give as high a signal output for that path as possible while preventing the received signal volume at a subscriber station connected to the terminal from going above the maximum comfort value for a listening subscriber.

The various objects and features of the invention are explained in detail in the following complete description thereof to be read in conjunction with the accompanying drawings in which:

Fig. l shows a single line functional schematic of a radio control terminal ofthe general type to which the invention is applicable;

Figs. 2 and 3 in combination show schematically control equipment in accordance with one embodiment of the invention applied to such a control terminal; and

Figs. 4 and 5 in combination show control equipment in accordance with an alternative embodiment of the invention applied to a control terminal of the type illustrated in Fig. l.

In the functional schematic of Fig. l each solid line represents a two-wire transmission path, and the transmission apparatus associated with each transmission path is illustrated by diagrammatic representations of their functions or appropriately labeled boxes. Contacting arrowheads in a transmission path indicate that the path at that point is normally made or enabled, and separated arrowheads in a transmission path indicate that the path at that point is normally broken or disabled. An arrow directed toward a make point (contacting arrowheads), in a transmission path from a box indicates that the path will be disabled by operation of the apparatus represented by the box and an arrow directed toward a break point (separated arrowheads) in a transmission path from a box indicates that the path will be enabled by operation of the apparatus represented by the box.

The control terminal of Fig. l is essentially a four-wire terminating circuit for a radio telephone system. As indicated, this four-wire circuit comprises a transmitting circuit TC leading to a radio transmitter and a receiving circuit RC leading from a radio receiver. The input of the transmitting circuit TC and the output of the receiving circuit RC are connected in substantially conjugate relation with each other and in transmission relation with the two-way circuit or line section L by a hybrid coil H and associated balancing network N in well-known manner. The two-way circuit or line section L may connect to a telephone switchboard in a normal two-wire telephone plant (not shown). The transmitting circuit TC includes in its input a transmitting 'volume control VCT and the following transmitting amplifier TA. The receiving circuit RC includes the receiving volume control VCR and the following receiving amplifier RA. Associated with this four-wire circuit is a voice-operated switching device or vodas VS for suppressing echoes and preventing singing, including a transmitting branch TS having its input connected across the transmitting circuit TC at some point A between the output of the transmitting amplier TA and the radio transmitter, and a receiving branch RS having its input connected across the receiving circuit RC at a point B in front of the receiving amplier RA. A portion of the receiving volume control VCR connected in front of the point of connection of the receiving vodas branch RS to TC is effectively in the input of the receiving vodas branch RS and may be used as a sensitivity control SCR for the receiving amplifierdetector RAD in that branch. The transmitting vodas branch TS includes the transmitting amplifier-detector TAD and the sensitivity control SCT therefor in the input to TAD.

In the absence of outgoing speech signals received from the two-way circuit L, the transmitting circuit TC is disabled or blocked at the point C beyond the point A of connection of the transmitting vodas branch TS thereto, as indicated, and the receiving circuit RC is operative to transmit speech signals incoming from the radio receiver to the receivinCr amplier RA and to the input of the receiving branch RS of the vodas VS, as indicated. Outgoing speech signals received over the two-way line L from a telephone subscriber will be impressed by the hybrid coil H on the input of the transmitting circuit TC and will pass thereover through the transmitting volume control VCT and the transmitting amplier TA towards the radio transmitter. One energy portion of these signals diverted at the point A into the transmitting vodas branch TS will pass through the transmitting sensitivity control SCT to the transmitting amplifier-detector TAD in which it will be amplified and rectified, and the rectified signal energy in the output 0f the latter will cause the transmitting circuit TC to be enabled at the point C and the receiving circuit RC to be disabled at the point D` This will permit the other energy portion of the outgoing speech signals in the circuit TC to be transmitted, with the amplification provided by the transmitting amplier TA, to the radio transmitter for radiation thereby.

While the circuit RC is maintained disabled at the point D by operation of the transmitting amplifier-detector TAD, speech signals or noise energy thereafter received over the circuit RC from the radio receiver will be prevented from entering the receiving vodas branch RS or from being fed back through the output of receiving circuit RC, hybrid coil H and transmitting circuit TC to cause false operation of the transmitting vodas branch TS or to set up a singing condition around the four-wire circuit.

If the speech signals of a distant subscriber are received over the circuit RC from the radio receiver during a time interval in which that circuit is held enabled at the point D, due to nonoperation at that time of the transmitting ampliner-detector TAD by outgoing speech signals, the main portion of the received signals after passing through the receiving volume control VCR and being amplified in the receiving amplifier RA will be passed from the output of the receiving circuit RC through the hybrid coil H to the two-way line section L over which it will be transmitted to the listening telephone subscribers line connected thereto through the associated telephone switchboard. Another energy portion of the received speech signals will be diverted from the receiving circuit RC at the point B into the receiving vodas branch RS in which it will be amplied and rectied by the receiving amplifier-detector RAD. The rectied signal output of RAD will cause the transmitting vodas branch TS to be effectively disabled at the point E in its output. Thus, while RAD remains operated by the applied received speech signals, subsequent operation of the transmitting amplifier-detector TAD by outgoing speech signals from the line section L or by echoes of received speech signals incoming from the radio receiver, due to the eiective disablement of TS at the point E, cannot result in the disabling of the receiving circuit RC at the point D to cut off the received signals, or the enabling of the transmitting circuit TC at the point C to permit the outgoing speech signals or echoes to pass to the radio transmitter.

Proper operation of such a control terminal under all service conditions requires adjustment from time to time of the transmitting volume control VCT in the signal transmitting circuit TC, the receiving volume control VCR in the signal receiving circuit RC, the transmitting sensitivity control SCT in the input of the transmitting vodas branch TS and the receiving sensitivity control SCR in the input to the receiving vodas branch RS. The functions of each of these controls will now be described.

The transmitting volume control VCT is provided to enable adjustment of the signal input to the radio transmitter to a relatively constant volume in order to fully load the radio transmitter for all subscribers Without excessive signal distortion regardless of whether the talkers speech is strong or weak, and whether the connecting land line to the talkers station is long or short, and to maintain the signal-to-noise ratio for the radio portion of the circuit as high as possible. This control is adjusted by the technical operator frequently during a conversation to maintain a normal reading on a volume indicator (not shown) connected across the transmitting circuit TC.

The transmitting sensitivity control SCT is provided to enable adjustment of the sensitivity of the transmitting amplier-detector TAD in the transmitting vodas branch TS as high as practicable to avoid clipping of outgoing speech while substantially preventing false operation of the transmitting vodas branch by land line noise. The correct setting of this control would be determined by monitoring on this vodas branch and it should be adjusted approximately once for each conversation. The receiving sensitivity control SCR is provided to enable adjustment of the sensitivity of the receiving vodas branch RS as high as possible consistent with preventing false operation of this branch by incoming static from the radio circuit. This is necessary to insure that the highest permissible receiving volume is delivered to the local subscriber for any particular setting of the transmitting volume control VCT and the transmitting sensitivity control SCT. The correct setting is determined by the technical operator by monitoring on this vodas branch between calls and noting false operations of the receiving amplier-detector RAD by static.

The receiving volume control VCR is provided to enable adjustment of the attenuation in the receiving circuit RC mainly to maintain the received signal volume-delivered to the local subscriber at the maximum value consistent with satisfactory echo margin. The echo margin requirement may be determined as follows. The sensistivity of the receiving amplifier-detector RAD, as seen from the radio receiver, should always equal or exceed the sensitivity of the transmitting amplier-detector TAD, as seen from the radio receiver, so that noise or speech energy incoming from the radio receiver is either of too low amplitude to result in the operation of either amplifier-detector or will operate the receiving ampliiier-detector RAD to cause the transmitting vodas branch TS to be disabled at E in the manner previously described. Consequently, the transmitting amplier-detector TAD should never be operated by echoes of the speech signals or noise incoming from the radio receiver transmitted into the signal transmitting circuit TC through the hybrid coil H, and the received speech signals should not be self-mutilated. 'I'he margin by which this requirement is met is called echo margin and may be defined as the difference, expressed in decibels, between the volume delivered by the control terminal to the two-wire line section or circuit L under a given set of service conditions and the maximum volume which could be delivered thereto by reducing the loss in the signal receiving circuit RC without producing echo operation of the transmitting vodas branch TS. It may be calculated from the following formula:

Echo margin=SRSTiLR+LT in decibels (l) where Sa=the sensitivity of the receiving amplierdetector RAD as measured at the radio receiver; ST=the sensitivity of the transmitting amplierydetector TAD as measured from the point A; and LR-i-LT=the net loss around the loop from the radio receiver through the signal receiving circuit RC, the hybrid coil H and the signal transmitting circuit TC to the point A.

The adjustments of three of the above-dened controls, i. e., the transmitting volume control VCT, the transmitting sensitivity control SCT and the receiving sensitivity control SCR, are dependent on factors Within the power of the technical operator to control. The adjustment of the receiving"- volume control VCR; however, is depend`- entrupon the adjustmentsotheother three conf trols. In addition tothe need for meeting the echo margin requirement. discussed above; for.' proper operation of the control terminal,v itfis;

necessary tolimit the volume of the receivedzsige nal energy delivered to alistening.subscriberfrom the signal receiving circuit, .RC through'. the" hy'- brid coil H, Athe twowirelinesection L and an`as;

sociated switchboard to'a maximum'value which'. will not befirritatingto: the ear'ofithat subscriber' whenreproduced by. his telephonefreceiver: The; proper relative 1 adjustments of the volume con-- trolszVCr'and'VCa and the sensitivitywontr'ols.

by the'r dot-'dash box' designated CP,- including anv arrangementof variableattenuators in :the transmission paths, and a mechanical linkage therebetween for automatically adjusting. the' netloss ofthereceiving circuit RC of the terminal to the' desired value to meetiboth of the aboverequirements.` Theelements of the control terminal of Fig.Y 2 corresponding to the same n elements in'thev diagrammatic showingofFig.' l are designatedv by'the' same reference charac# ters.l A detailed showing f a mechanical linkageon .the control panel'CP in accordance with oneV embodiment of: the invention is given, in Fig. 3 to be'described in .conjunction with Fig. 2.

The transmitting circuit TC of the radio con-- trol terminalof'Fig. 2 includes in order fbetween the hybrid coil Hand the radio transmitterRT, reading from left to right: the manually-con,- trolled variable resistancey attenuator Al;; the transmittingamplier TA; the hybrid coil Hl and associatedbalancingnetwork Nl for connecting the input of the transmitting vodas branchTS in receiving relation with-respect to the output of the transmitting ampliner TA and in substantially conjugate relation with the output portion of TC leading to the radio. transmitter RT; andthe normally closed short-cir.- cuiting connection Cl.across the circuit .TC at the point C, which, whenclosed, is.v adaptedto eiectively disableA that circuit. at that point. The receiving circuit RC of the controlterminal includesv between the radio receiver' RR andthe hybrid coil H, reading from right to left, a nor mally open short-circuiting connection C2 across the circuit RC at the point'D,-which, when closed, is adapted to effectively disable the circuit RC at that point; the variable resistance attenuator A2`connected between the points D and B; the variable resistancel attenuators A3 and A4 connected in tandem in the circuit RC beyond the point B1; and the receiving amplier RA.

The'transmitting vodas branch TS includes a manually-controlled variable resistance attenuf ator `A5lin its input; the following'v transmitting amplieredetector TAD; and the switching relays Rl andiRZV connectedin parallel to', the output of TAD and operatively responsive to the rectified output of the latter when voice signals areapplied to; its input, toirespectively open theshort-circuiting connection Cl across the circuit' TC at thepointC'to enable that circuit, andv to close the.` short-circuiting con. nection C2 acrossA tliezcircuitRCatzthe point -D r, ceiving sensitivity control SCR of Fig. l.

to effectivelyydisable thelatter circuit at that point. its input connected across the receiving circuit RCv at the point B includes the receiving ampli-v er-detector RAD and the switchingA relay- R3 operativelyl responsive to the rectified output of the. latter when voice signaling energy is applied to its. input from the receiving circuit RC, to closev the shortfcircuiting connection C3 across the output of the transmitting vodas branch TS at the point Eto effectively disable thetranse mitting switching relays Rl and R2.

Thel variable attenuator A'lv with its control,

shown in Fig. 3, operates as the transmitting j volume.: control VCT of Fig. 1.

resistance-type the lossvalueof which in the.

circuit inV whlch'it is connected is adapted to bechanged in steps by simultaneous adjustment of the two movable control arms of the network to cut in or outy resistance in the 'series and shunt branches of the network, respectively. In the practical embodiments. of` the invention to' be described, the range of adjustments of the variable attenuators Al, A2, A3, `All andgAS are 0-45, 0-50, (L55, 0-101 and 0-10 decibels, respectively, each step oft adjustment for the-attenuatorsgAI, A2, A3, AAandAS equaling 2.5 decibels.

Level tables for thev proposed semiautomatic radio controliterminal in accordance with the invention shown in Fig. 2 are given below. The assumptions for this terminal are: a speech volumey of +6VU to the radio transmitterA RT;

a speech volume of -l-lOVU fromthe radiofreceiver RR;V an assumedtalker volume Vr'from theflandlineL` in the range fromV -29VU to +16VU; a maximumreceived volume Va. dclivered tothe' land lineLof` -9VU;'a` gaincf 39 decibels; fory the-transmitting amplifier TA andofAO decibels for the receiving amplifier RA;ga.maximum sensitivtyST forl the transmitting, amplifier-detector TAD as seen from the radio transmitter vRT- vof 25 decibels (i. e., it will operatefor a1000-cycley tone of -25 dbm) which can be reduced by 10 decibelsby adjustmentofthe attenuatorAS; amaximum sensitivityY Sa. for the receiving amplifier-detector RADat the input ofthe receivingvodas branch RSfof 44- decibels (i. er, it will operate'forsa G-cycle tone of Jie dbnl) which can be reduced byl 50'decibels by adjustment'of` theattenuator. A2; a; return. loss, at' the land line L of 0-decibels; a'lossacrossthe.hybridxcoil H from the receiving circuit RC to the .transmitting amplifier-detectonTADwhen eachof the attenuators A'l to A5fare: adjusted to provide() decibeliloss, of -'v71 decibels (combined'gain of ampliers. TA and RA of 79 decibels minus hybridcoil lossiof 8 decibels). Inthe tables below the adjusted settings of attenuators'Al, A2,.A3 and-*A4 are given in decibels under correspondingly labeled headings.. The echo margin EM is computed from Formula 1.

The receiving vodasbranch RS having Thev variable.

9 in decibels. The tables show the eight extreme conditions, maximum and minimum, of VT, ST and Sa.

l@ afixed to the opposite faces of the disc II at a mean distance radially from shaft 8 approximately equal to the mean distances radially of the ve A5 sn sT A1 412 A3 A4 o EM 46 A3 A4 (VU) (ab) (ab) (ab) (ab) 1111) (ab) (db) (db) (ab) (VU) It Will be noted from the above table that there are four cases, (l), (2), (3) and (4), Where the receiving volume Va is -9VU. If

stop pins Id and I5 from shafts 'I and 8, respectively, and extend in a direction parallel to that of shafts 'l and 8 to given points between discs I and from shaft T. Similar stop plates I6 and I6' are the sum of the losses of A2, A3 and All can 20 II, and between gear I 2 and disc II, respectively, never be 55 decibels, its receiving volume Va such that for certain rotated positions of the in these four cases will be held to -9VU as seen discs IG and II, the pin Ill will bear against the from the following table: stop plate I6, and for certain rotated positions VR= (gli) (15 153 15 it (fig) 101 gli?) +46A15,"2)A3*A4 +16 0 44 25 +45 o +10 +45 71 +48 9 29 10 44 15 0 0 0 +55 71 +13 9 +16 10 44 15 +45 0 o +55 7 +58 9 +16 10 s 15 +45 +50 o +5 71 +8 9 One mechanical linkage for controlling the adof the disc I I and the gear I2, the stop plate I6 justments of the attenuators AI to A5 to accom- 35 will bear against the stop pin I5. The spring I3 plish the purposes of the invention is shown in serves to hold pin I4 and plate I6 in contact so Fig. 3. It includes a shaft I suitably mounted on that the motion of shaft 'I is transmitted to shaft panel CP, fixed at one end axially to the receiving 8 unless pin I5 prevents it. sensitivity control dial 2 and at the other end The movable arms of the variable attenuator axially to the gear 3 so that rotation of dial 2 in im A2 in the receiving circuit RC of Fig. 2 are so con- `one direction or the other produces acorrespondnected to the shaft I that when the dial 2 is ing rotation of gear 3. A second shaft 4 mounted turned clockwise or counter-clockwise (as seen as shown on panel CP in parallel with the shaft from the front of panel CP) by the technical 0p- I, is xed at one end axially to the transmitting erator to increase or decrease the receiving vodas sensitivity control dial 5 and at the other end 45 sensitivity to the desired value, the loss value of axially to the gear so that rotation of dial 5 in that attenuator in RC is decreased or increased, one direction or the other will produce a correrespectively, by the same amount, and the gear 3 sponding rotation of gear 6. rotates in the same direction a corresponding A third shaft l and a fourth shaft 8 are amount. The movable arms of the attenuator mounted end to end on panel CP as shown, in 5U A5 in the transmitting vodas branch TS and of parallel with the shafts I and 1i. The transmitthe attenuator A3 in the receiving circuit RC are ting volume control dial 9 is aiiixed axially to one reversely connected to the shaft l sc that when end of the shaft l farthest from shaft S, and a disc the technical Operator turns the dial 5 Clockwise #d is afiixed axially to the shaft 'l at an inter- (as seen from the front cf the panel CP) to inmediate painlr thereon so that rotation of the dial 55 crease the sensitivity of the transmitting vodas 9 in one direction or the other will produce a corbranch TS to the desired value, the loss value of responding rotation of the disc lil. A second disc attenuator A5 in TS is reduced Vand the loss value Il identical with disc IQ, is axed axially to the of the attenuator A3 in RC is increased by the shaft 3 at an intermediate point therealong and a same amount; and When the tehrlcal OperatOI gear I2 is mounted axially in bearings near the 51; turns dial 5 counterclockwise the loss value of atend of shaft S farthest from shaft 'i so that the tenuator A5 in TS is increased and the loss value latter gear is adapted to rotate freely on shaft 3. Of the alltelluatOr A3 in RC iS redlled by the The free ends of the shafts i and 8 are held in juxsame amount. taposition by the bearings of each shaft, and a The movable arms of the attenuator AI in the helical spring I3 encircling the extensions of the transmitting circuit TC and the movable arms two shafts I and 8 is affixed at its opposite ends to of the attenuator All in the receiving circuit RC the near faces of the discs I G and l l, respectively. are reversely connected to the shafts 'I and 3, re- A stop pin it is affixed to the face of disc if) nearspectively. Thus, when the techni-cal operator est disc l2 at a point near the periphery of disc lll, turns the transmitting volume dial S clockwise and extends in a direction parallel to that of shaft 7o (as seen from the front of the panel CP) to raise 'I tc a given point between discs I@ and Il, Asimithe outgoing transmitted volume, the loss value lar stop pin is affixed to the face of the gear of attenuator AI in TC is decreased and, because I2 nearest disc l i at a radial distance from shaft of the stop pin I@ of disc I0 bearing against stop 8V approximately the same as that of stop pin It plate I6 of disc Il positive motions are imparted 75 to shaft 3 and thus to the movable arms of atgears II, I8, I9 'and29. `attached by its axialshaft 2l axially to the gear 22--which meshes with thegear 3. The bevel gear V26 is attached by its axial shaft 23axially -to the tenuator A4-such that'the loss valueof the latter Vattenuator in RC is increased step yby step in the fsame amounts. `Similarly, when the technical operator turns the transmitting volume control dial 9 counter-clockwise to reduce the outgoing transmitted volume, the loss value of the attenuator AI in TC is increased. The spring I3 has enough tension holding the stop plate I6 of Ydisc II against the stop pin I4 of disc I0 s0 that the shaft 8 is also rotated so as to cause the loss of the attenuator A4 in RC to be decreased step by step in the same amount as the loss of the attenuator AI in TC is increased unless the action'of pin I5 prevents it. The various elements of the arrangement of 3 controlling the adjustments of the attenuators AI and A4 are designed and relatively arranged so that when pin I4 is touching plate I6 vthe sum of the losses of attenuators AI andA4sis 45 decibels, i.-e., when theloss of AI is zero the lossof A4 is 45 decibels, and when the loss of AI is ..25 decibels the loss of A4 is 20 decibels, etc. `This `isdn spite. of. the fact that the attenuator A4 may provide a total loss of 55 decibels. The above tables show this clearly. As shown, the stop pin I 5 of gear I2 does not engage stop plate IB of disc I I, so each dial 2, 5 and-9 controls its ownattenuators as described with no linkage between these dials.

The phase between the gearf|2 carrying stop pin I5, and gear 3 and between gear I2 and gear 6, must be so adjusted that when the algebraic sum of the losses inserted by attenuators A2, A3 and A4 into the receiving circuitRC in response to manual'operation of the dials 2, 5 and 9 by the vtechnical .operator equals the. minimum value which-:will restrict thereceiving volume delivered by the control terminal over Atheline L to the make :contactwith kthe stop plate I6. particular lossvalues of the attenuatorsand gains ,of the: amplifiers used and the assumed incoming and outgoingspeechlevels, this minimum value vfor the net less in the receiving circuit is 55 decibels. The .proper phase adjustment is produced by the -set of gearsconnecting gear I2 to gears `3 and 5. This setof gears includes the differentiall gears consisting of the four` meshing bevel The bevel gear I9 is gear 24 meshing with the gear I2. A gear25 .freely rotatable in bearings on the shaft 23 meshes with thegear G. vThe bevel gears II and I3..are.idler gears rotatable freely in bearings on the commonA shaft 26 rigidly supported at its ends bythe members'ZI and 28 in turn rigidly attached to the face of the `gear 25 at. respective points `en uidistant from the axis of rotation of the latter.

Each .of the gears I2,.2\4, 22 and 3 has the Same number of teeth. Each of the bevel gea-rs I'l, I8, I9 and 429 also has `the same number of. teeth. The gear `hasone-.half -as many teeth as .gear

Itis assumed that all of the variable afttenuators AI to` A5 have the samenumber of degrees of rotation for 2.5 decibel steps of loss.

When the receiving sensitivity control dial V2 is rotated in the clockwise direction '(as seen from the front of the panelCP), the loss of attenuator A2 in RC will beV decreased to effectively increase the sensitivity of the receiving vodas rbranchRS, and the 'gear 3 will be rotated clockwise by the same amount .producing a correspending clockwise rotation Aof gear 22 and of the amount. Ying counter-clockwise rotation of the gear 24 bevel gear I9 through shaft'ZI. 4This will vproduce a corresponding rotation of the idler bevel gears I and I8 on shaft 26, meshing with the gear I9, one in the clockwise direction and the other in the counter-clockwise direction. Assuming that the shaft IZB is held in a fixed position because of nonerotation at the time of the transmitting sensitivity control dial 5, the other bevel-gear 2i) meshing with gears I'I and I8 will be rotated counter-clockwise by a corresponding This motion will produce a correspondthrough shaft 23 and corresponding clockwise rotation of the gear I2 Von shaft 8. Thus, as the receiving vodas branch sensitivity is increased by the decrease intheloss of theattenuator A2 in vthe input of RC with the turning Yof the receiving sensitivity dial 2.clockwise, the clockwiserotation of gear I2, due to the stoppin I5 bearing against thestop kplate IB, will cause the disc II theshaft 8 and the movableV arms ,ofthe attenuator All connected to thatshaft to be moved clockwise so as to increase the loss of the latter attenuator, decibel for decibel, with the decrease in loss of attenuator A2 and thus maintain the ralgebraic sum of the losses of the attenuators in RC at the required minimum value (55 decibels). On the other hand, if before the engagement of the stop pin I5 on gear I2 with the stopplate -IG on disc II, the transmitting volume is re- `from,theattenuator A4 in the receiving circuit 'rotatedcounter-clockwise by the same amount.

This, through rotation of the gears 212, I9, IT, I8, 20 and 24 in the manner previously described but in the opposite directions will result in the Acounter-clockwise rotation of gear I2 on shaft 8. Unless thestop pin I4 of the disc I0 bearing Yagainst `the stop plate I6 on disc II under control of adjustment of the transmitting volume control dial 9 prevents it, the stop pin I5 bearing against stop plate I5 of disc II will cause the disc II, shaft 8 and the vmovable arms of Athe attenuator A4 to be moved counter-clockwise so as to reduce the loss of the latter attenuator, decibel for decibel, with the increase in the lossof the attenuator A2 thus maintaining vthe algebraic sum of the losses of the attenuators in RC at the required minimum value (55 decibels).

If, with gear I9 stationary due to non-rotation of the receiving sensitivity control dial 2, the transmitting sensitivity control dial 5 is rotated in the clockwise direction to reduce the loss of ythe attenuator A5 in the transmitting vodas branch TS increasing the sensitivity of the latter branch, and .to increase the loss of the attenuator A3.in the receiving circuit RC by the same amount, the corresponding clockwise angular rotation of the gear `I will produce a clockwise rotation of the gear 125 over half that angle (due -to the fact that there are twice as many teeth in gear as there Iare teeth in gear E). Because of the fixed .attachment `of the supporting members 2'! and 28 for shaflt 26 to the face of gear 25, this will c-a-use Ithe tilting of the idler shaft 2'6 and the gears I'I Iand I8 rotatable freely thereon clockwise over -a corresponding angle. This will produce a clockwise Iangular rotation of the gear y2l) mesh-ing with -gears I'I and I8 and thus of the gear 2i connected through shaft 23 to the gear 20, which in turn will cause :a counterclockwise rotation `of the gear I2 meshing with gear 2d. If pin I 5 at this time is in contact with plate I6', this will cause the attenuation of Al! to be reduced unless -pin Id bearing against stop plate I6 prevents it. If pin I5 is not at this time in contact with plate I6', the only effec-t is to rotate pin I 5.

If the transmitting sensitivity control dial 5 is rotated in the counter-clockwise direction to increase the loss of attenuator A5 and thus to reduce the sensitivity of the transmitting vodas branch, and to decrease the loss of the vattenuator A3 in RC by the same amount, the corresponding counter-clockwise angular rotation of gear G will produce a counter-clockwise rotation of gear @25 over half that angle. This, through supporting members 2l and 2-8 for shaft 6, xed to the face `of gear 25, will cause the tilting of the idler shaft .26 and the gears I1 and I8 freely rotatable thereon counter-clockwise over a corresponding angle. The resultant rotation of gears 2D, 2d and 26 in the manner previously described but in 'the opposite directions will finally result in a clockwise rotation of the gear I2 on shaft 8. If pin I5 is at this ltime in contact with the stop plate I6 this will cause the loss of vattenuator Ai to be increased unless the pin i4 bearing against stop plate I6 under control of the transmitting volume control dial 9 prevents such adjustment. If pin I5 at the time of rotation of gear I2 is not in contact with the stop plate I6' the only eiect will be the rotation of pin I5.

It is apparent that there must be enough friction in the three dials 2, 5 and 9 so that when the stops engage, the spring i3 does not turn the dial which is supposed to be at rest. The spring I3 must be strong enough, however, to insure that the adjustment of loss in the attenuator All in RC follows the adjustment of loss in the attenuator AI in TC when there is no engagement, i. e., the spring I3 must be able to move the movable arms of the attenuator Ali. This means a marginal. requirement on the tension of the spring I3 and a minimum amount of friction in the three dials 2, 5 and 9. Alternativeiy, the controls for the variable attenuators AI and A2 may be provided with detents having considerable friction `and that for attenuator A4 with no detent. Spring I3 must then be made strong enough to turn the movable arms of attenuator At but not strong enough to overcome the detents.

The equipment on the control panel CP in accordance with the invention illustrated in Figs. 4 and 5 differs essentially from that illustrated in Figs. 2 and 3 merely in the use oi a diiierent arrangement for taking care of changes in transmitting sensitivity and the use of a diiierent gearing arrangement for coupling gear 3 to gear I2. This alternative equipment will now be described.

As shown in Fig. 4, two 5-decibel attenuators kA6 and Al connected in tandem and controlled electrically by the switching relays R4 and R5,

respectively, are substituted for the single mechanically-controlled variable attenuator A5 (Fig. 2) in the input of the transmitting vodas branch TS, and two 5-decibel attenuators A8 and A9 connected in tandem and controlled electrically by the switching relays RE and Rl, respectively, are substituted for the mechanically-controlled varin able attenuator A3 (Fig. 2) in the receiving circuit RC between the point B and the input of the receiving amplifier RA. The operating winding of relay R4 is connected in series between the (AVE) Contact 29 of a switch SI associated with the transmitting sensitivity control and the positive terminal of the battery 39 having its negative terminal electrically grounde l. The operating winding of relay R5 is connected in series between the (MIN) Contact 3| of switch Sl and the positive terminal of battery 3i?. The operating winding of relay R6 is connected in series between the (AVE) Contact 29 of switch SI and the positive terminal of battery 32 having its negative terminal electrically grounded. The operating winding of relay R'l is connected in series between the (MTN) contact 3| of switch SI and the positive terminal of battery 32. The contacts 25 and 3l are arranged in an arc having a mean radius equal to the distance from the pivoting point of switch arm 4i to the center line of the arcuate wiper 33 attached to the other end of arm Lil.

As shown in the sleeve 3d mounted to rotate in the bearing 35 loosely surrounds `the shaft i to which the receiving sensitivity control dial 2 and the movable arms of the attenuator A2 in the input of the receiving circuit RC, are attached in the manner described previously in connection with Fig. 3. 'I'he gear 3, which in the arrangement of Fig. 3 was aiiixed axially to one end of shaft i, in the arrangement of Fig. 5 is aiiixed axially to the sleeve Sli so that it is rotatable therewith in the bearing 35. The independent shaft i and associated transmitting sensitivity control dial 5 and gear 6 utilized in the arrangement oi Fig. 3 are eliminated in the arrangement of Fig. 5 and are replaced by a. gear 36 mounted axially tightly on sleeve 34, a gear 37 meshing with the gear 36 and aiixed axially to a shaft 38, the shaft 38 being connected at one end through the detent mechanism 39 to the transmitting sensitivity control knob 4! which is mounted on the receiving sensitivity control dial 2 and rides around on it. The electrically grounded movable arm lll of switch Si, disposed at right angles to shaft 38 and having one end aflixed to the other end of that shaft is adapted to be rotated by that shaft when the transmitting sensitivity control knob iii is rotated in one direction or the other. The movable arm 4i is shown as electrically insulated from shaft S3 but does not need to 'be so insulated if all gears are assumed to be grounded. The arm 4I terminates in an arcuate metal wiper 33 which as the arm is rotated sweeps over the metal contacts 29 and 3! to successively connect electrical ground to these contacts.

The train of gears II to 28, 22, 2t and 25 and their associated shafts and bearings used in the arrangement of Fig. 3 for providing the desired coupling between gear I .'i and gear I3 and between gear l2 and gear are eliminated in the arrangement of Fig. 5 which employs the idler gear l2 connected between and meshing with the gears 3 and I2.

When the transmitting sensitivity control knob 40 is held in one extreme position of adjustment,

the arcuate wiper 33 on movablearmM of switch .Si is. heldout ofcontact with the switch contacts .29.and 3i, and no energizing circuits are closed from vthe battery .30 or 32 for the relays R4 to Rlcontrolling the adjustment of the attenuators A6 to A9. In that case, the contacts of the unenergized relays Rt to RT .will be in the positions .indicated in Fig. 4 in which the series .resistance arms of attenuators A6 and A1 are short-circuited andthe shunt resistance arms of these attenuators are open; and the shortcircuiting connections around the'series arms of attenuators A8 and AS are open and the shunt resistance arms of the latter attenuators are closed. attenuators A6 and A? will insert substantially noloss in the input of the transmitting vodas branch TS so the latter has its maximum sensitivity and .the attenuators A8 and A9 insert a maximum loss (1G decibels) in the receiving cir- .cuitRC to maintain the required echo margin around the loop.

When the technical operator turns the transmitting sensitivity control knob counter-clockwise (as seen from the.v front of panel CP) sufficiently'so that the arcuate wiper 33 of arm il rests on vthe (AVE) contact 29 of switch Si, energizing circuits are closed for relays R4 and R6 from batteries Eiland 32, respectively, causing the operation of these relays. The operation of relay R4 causes its back contacts to be opened lto remove the normal short-circuiting connection across theseries resistance arms of attenuator A6 and its front contacts to be closed to close Athe normal open'circuit in its shunt resistance arm, thus effectively inserting the loss (5 decibels) of attenuator A6 into the input of the transmitting vodas branch TS to reduce` its sensitivity by va corresponding amount. The operation of relay. R6 will causev its back contacts to be closed .to connect-a short-circuiting connection across theseriesresistance arms of the attenuatcr A8 and its front contacts to be opened to-open the shunt resistance arm of that attenuator. Thus the loss of 5 decibels normally insertedin the receiving circuit RC by attenuator A8 will be effectively removed from that'circuit to compensate for the reduced sensitivity Yof-the transmitting vodas branch TS and maintain the echo margin.

Similarly, when the technical operatorturns the transmitting sensitivity control knob 40 'counter-clockwise to the point where the arcuate wiper 33 of switch arm 4| makes contact with the (MIN) contactl on switch Si, energizing cr- .cuits-will beclosed for switching rela-ys R5 and R7 vfrom batteries 30 .and 32, respectively. The re- .sultant operation `of relay R5 will open the normally closed back contacts of this relay and close .-itsnormally open back-contacts to respectively .remove the rnormal short-circuiting connection around the series resistance arms of attenuator A1 and close the shunt resistance arm thereof, so that the 5-decibel loss ofthe attenuator A1 iS inserted into the input of the transmitting vodas branch TS. The resultant operation of relay R1 `will cause the back contacts of that relay to be closed and the front contacts thereof to be Opened to respectively short-circuit the series-resistance arms of .attenuator A9 and close its shunt resistancearm, thereby effectively removing the normal -decibel loss of attenuator A9 from vthe receiving circuit RC. The arcuate wiper .i3-of .switcharm 4| .isof suchlength thatfor .this .condition .of .rotation of the .transmitting sensi- In that condition of the switch SI, the i.

4tivity control knobw, the .wiper 33 rests On the tenuators AS and AT-is inserted in the transmitting vodas branch TS to lower. its sensitivity bylO decibels and the 5-decibel loss of each of theattenuators A8 and A9 is effectively removed from the receiving circuit RC so that the loss ofthe latter circuit is decreased by 10 decibels to give as high a received volume as possible to theland line L and still maintain echo margin.

As in the case of the similar controls of Fig. 3 described previously, when the technical operator turns the transmitting volume control dial 9 clockwise or counter-clockwise, the corresponding rotation of the shaft 'l controls the variable attenuator Al in rTC so as to cause itsattenuation -value tofbe decreased or .increased respectively, to adjust the transmitting volume in TC to the desired value and, because of the mechanical couplingprovided between the shafts and 8 by the spring I3, the stop pin Hlv on disc. |13 attached to shaft 1 and the stop plate i6 on disc l l attached to shaft 8, shaft 8 will be rotated tocontrol the variable attenuator A4 to increase o1' decrease, respectively, the loss in RC in corresponding amounts so as to maintain echo margin.

If the transmitting sensitivity control knob 40 is left stationary, then when the technical operator turns the receiving sensitivity control dial 2 and thus shaft i.clockwise or counter-clockwise to adiust attenuatcr A2 in the input of the receiving circuit RC so as to bring the sensitivity of the vodas receiving branch RC to the desired setting, sleeve,34 and therefore gear.3 attached thereto will be rotateda corresponding amount in the same direction dueto the mechanicalcoupling betweensleeve .34 .and dial 2 provided by the meshing-gears 36 and .31, shaft38 and detent mechanism 39. If then, with the receiving sensitivity control dial 2 stationary, suppose the technical operator rotates the transmitting sensitivity control knob40 ondial 2, and thus shaft 38 in one direction :or the other to a different position so as to electrically control through switch SI, as previously described. the loss values of the attenuators A6 and A1 in the input of the vodas transmitting branch TS to adjust the sensitivity of that branch to the desired value, and the loss values of the attenuators A8 and A9 in the receiving circuit RC to provide a compensatingT loss change in that circuit to maintain echo margin. Then, the sleeve 34 and thus the gear 3 attached thereto will be rotated a corresponding amount in the opposite direction due to the mechanical coupling providedbetween that sleeve .34 and knob .40 by the meshing gears 36 and 31.

the shaft 38 and the detent mechanism 39. Thus a desired phase shift has been introduced between shaft l and sleeve34, corresponding to the phase shift introduced by the differential. gearing in the arrangement of Fig. 3.

.The rotation of the gear 3 for both the vodas receiving branch sensitivity and the vodas transmitting branch sensitivity adjustments are transmitted into the same rotation of a corresponding amount in the gear I2 rotating freely on the shaft 8, dueto the couplingof gear through idler gear 42 to gear I2, The main function of the idler gear 42 is to insure that the rotations of the dials 9. andy 2 are in a desirable direction, i. e., so that the clockwise rotation, from the front of panel CP,.of transmitting volume control dial 9 will adjust theattenuator. AI so asto increasethe transmitting volume in TC and the clockwise rotation of the receiving sensitivity control dial 2 Will adjust the attenuator A2 in the input of RC so as to effectively increase the sensitivity of the vodas receiving branch RS.

By suitable design of the mechanical coupling between the sleeve 3d and the transmitting sensitivity control knob d@ and the receiving sensitivity control dial 2, the phase shift between the gear l 2 and gear 3 is adjusted so that when the algebraic sum of the losses inserted by the variable attenuators A2, All, A8 and A9 in the receiving circuit RC in response to manual operation by the technical operator of dial 2, dial S and knob 40, respectively, equals the minimum value (55 decibels for the particular values of the attenuators, the particular gains of the ampliers used and the assumed incoming and outgoing speech levels), which will restrict the receiving volume delivered by the control terminal over` the line L to the listening local subscriber to the maximum comfort value, the stop l5 on gear I2 will just make contact with the stop plate I6 on disc il. Then, if the sensitivity of the vodas receiving branch RS is increased further by reducing the loss of attenuator A2 or the sensitivity of the vodas transmitting branch TS is decreased further resulting in further reduction of the loss of attenuators A8 and A9 in the receiving circuit RC, the resulting rotation of the gear l2, because of pin I5 bearing against stop plate I6', will cause the loss of the attenuator All in RC to be increased, decibel for decibel, to maintain the algebraic sum of the losses in the receiving circuit at the required minimum value (55 decibels). If before the engagement of the stop pin l5 with stop plate I6', the transmitting volume is reduced by turning the transmitting volume control dial 9 counter-clockwise the resultant rotation of shaft 8 due to the tension of the spring l 3 holding stop plate IB against the stop pin i4, will cauSe the loss of attenuator A4 to be reduced until the algebraic sum of the losses in the receiving circuit RC again is equal to 55 decibels at which point further adjustment `of attenuator A4 is prevented by the engagement of the pin i5 on gear l2 and the stop plate i6'.

Various modifications of the arrangements illustrated and described above which are within the spirit and scope of the invention will occur to persons skilled in the art.

What is claimed is:

1. In a control terminal for controlling signal transmission in opposite directions between a two-way line connectable to the subscriber stations of a voice frequency telephone network and the radio transmitter and receiver at one end of a two-Way radio telephone system, including one one-way amplifying path for repeating outgoing telephone signals received over said line from a subscriber station to said radio transmitter for radiation thereby, a second oppositely-directed one-way amplifying path for repeating incoming voice frequency telephone signals detected by said radio receiver to said line for transmission thereover to a subscriber station and a vodas switching circuit having a transmitting branch connected to said one path and controlled by the outgoing signals therein and a receiving branch connected to said second path and controlled by the incoming signals therein, for directionally controlling signal transmission through said terminal while suppressing echoes and preventing singing: means to insure proper operation of said terminal over a wide range of line and radio noise conditions encountered in service comprising three variable attenuators respectively connected in said one path and in the inputs to said vodas transmitting and receiving branches; three manual controls operable by the technical operator of the terminalgto separately adjust the loss value of a different one of said three variable attenuators so as to make the outgoing signal volume level in said one path and the sensitivities of said vodas transmitting and receiving branches, respectively, of the optimum values for the existing noise conditions in said range; variable attenuation means in said second path, comprising a plurality of tandem-connected portions; a mechanical linkage interconnecting different portions of said variable attenuation means with a diiferent one of said three manual controls in such manner that when the latter are adiusted to the desired settings the loss value of said variable attenuation means will be automatically adiusted to maintain satisfactory echo margin in said terminal and means for restricting the volume of the signals delivered b v said. terminal over said line to a subscriber station to a maximum value tolerable to the ear of a listeningr subscriber thereat comprising means for mechanically interlocking said different portions of said variable attenuation means so as to effectively add algebraically the losses inserted in said second path for any adjusted positions of said three manual controls and other mechanical means automatically responsive to a reduction in the net loss of said variable attenuation means to the minimum value which will restrict the volume of the signals delivered to the subscriber station to said maximum tolerable value, to prevent further reduction of the loss of said variable attenuation means.

2; The control terminal of claim 1, in which the one of said three variable attenuators controlling the sensitivity of said vodas receiving branch is located in said second path in front of the point of connection of that branch thereto, said variable attenuation means is located in said second path bevond the point of connection of said vodas receiving branch thereto. each of said nrianualv controls comprises a rotatable dial having a shaft axially affixed thereto. the shaft of each of said dials being mechanically connected to a different one of said three variable attenuators in such manner that manual rotation of a dial with its shaft by the technical operator in one or the opposite direction will increase or decrease, respectively. the loss value of the attenuator connected thereto, said mechanical linkage comprises means mechanically connecting the shafts of the two dials respectively controlling the attenuators in said vodas transmittingr branch ano! in said one path to different portions of said. variable attenuation means in said second path in such manner that when either of said two dials and its shaft is rotated in said one or said opposite direction so as to increase or decrease, respectively, the loss in said vodas transmitting branch or in said one path by a given amount, the loss value of said variable attenuation means in said second path is automatically decreased or increased. respectively, by the same amount, said interlocking means including a set of differential gears for mechanically coupling the shafts of the three manual controls.

3. The control terminal of claim 1, in which the one of said three variable attenuators controlling the sensitivity of said vodas receiving branch is located in said second path in front of the point of connection of that branch thereto, said variable attenuation means includes a fourth and a fifth variable attenuator connected in tandem in said second path beyond the point of connection of said vodas receiving branch thereto, each of said three manual controls comprises a separate rotatable dial having a shaft axially affixed thereto, each of the shafts of the three dials being mechanically connected to a different one of said three attenuators in such manner that manual rotation of a dial and its shaft in one or the opposite direction by the technical operator increases or decreases, respectively, the loss value of the attenuator connected thereto, said mechanical linkage comprises means mechanically connecting the shaft of one dial controlling the variable attenuator in said one path to said fourth variable attenuator and the shaft of a second dial controlling the variable attenuator in the input of said vodas transmitting branch to said fifth variable attenuator in such manner that the rotation of said one or said second dial in said one or said opposite direction to increase or decrease, respectively, the loss value of' said second or said third variable attenuator by a given amount, automatically decreases and increases, respectively, the loss value of said fourth or said fth attenuator, respectively, in said second path by the same amount, said means for interlocking said different portions of said variable attenuation means comprises differential gearing means interlocking said one Variable attenuator and said fifth variable attenuator with said fourth variable attenuator and said other mechanical means includes a spring and stop mechanism for effectively providing slippingl between the interlocked attenuators in said second path over the high volume portion of the range of adjustment.

4. The control terminal of claim 1, in which one of said three variable attenuators is inserted in said second path in front of the point ofY connection of said vodas receiving branch thereto so as to effectively control the sensitivity of that branch in accordance with its setting, said variable attenuation means includes a fourth and a fth variable attenuator inserted in tandem in said second path beyond the point of connection of said vodas receiving branch thereto, each of said manual controls comprises a rotatable dial having a shaft axially affixed thereto, the shaft of each of the dials being mechanically connected to a different one of said three variable attenuators in such manner that manual rotation of a dial in one or the opposite direction will increase or decrease, respectively, the loss value of the attenuator connected thereto in an amount proportional to the degree of rotation, said mechanical linkage includes means mechanically connecting the shaft of the one of said dials controlling the attenuator in said one path and the shaft of a second of said dials controlling the attenuator in said transmitting vodas branch to said fourth and said fifth variable attenuator, respectively, in such manner that when the loss value of the attenuator in said one path or of the attenuator in said vodas transmitting branch is increased or decreased by rotation of the associated dial the loss value of said fourth or said fifth variable attenuator is decreased or increased, respectively, by a corresponding amount to maintain echo margin in said terminal, said interlocking means includes gearing for mechanically coupling said one and said fifth variable attenuators with said fourth variable attenuator and said other mechanical means includes a spring and stop arrangement associated with said fourth attenuator for preventing its further adjustment when the net loss of said variable attenuation means has been reduced to said minimum value.

OWEN R. GARFIELD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,206,146 Wright July 2, 1940 2,224,569 Dickieson Dec. 10, 1940 2,273,945 Fisher Feb. 24, 1942 2,510,975 Hawks June 13, 1950 

